Optical glass fiber lines have proven to be particularly suitable for the low-loss transmission of information having high information density. Usually information signals in electrical form are converted into optical signals, for example, using light-emitting diodes or laser diodes, and they are coupled into a corresponding optical fiber-optic line. At appropriate locations in the network, the signal is detected, for example, using a photodiode and is once again converted into an electrical signal, such as can be further processed in the customary manner. This signal transmission is well suited for overcoming large distances. At suitable intervals in the appropriate lines, amplifiers and/or regenerators are inserted, which are designed to assure that the signal arrives at the signal sink configured, for example, by the photodiode in an easily receivable form. Just as in the case of electrical networks, it is necessary to provide nodes, through which signals are conveyed to a specific desired receiver and as a result of which it is possible to provide an alternative path for a main line path in the event that the transmission on the main line path is disturbed. As a result of appropriately provided bytes in an overhead of the useful signal to be transmitted, it is also possible to undertake automatic alternative line switching operations. One disadvantage in this method is that the switching operation of an alternative path is only possible within an established transmission standard for the useful signals and that, in a conventional system, an optoelectronic conversion of the signal is necessary at the ends of the segment that is protected by an alternative path. These ends do not necessarily coincide with the sources/sinks of the useful signals.
European Patent No. 0 440 276 describes adding, outside of the useful signal band, a communication signal, using optical couplers. As a result, control and command signals can be transmitted between the nodes of the transmission device. Whereas the transmission of useful signals takes place in the so-called “third window,” the “second window” has been provided for the transmission of communications signals. The “windows” arise from from the damping characteristics of the glass fiber material for certain wavelength ranges. In the “third window,” the damping is minimal, whereas the “second window” is formed using a different damping minimum, in which the lower damping values of the “third window” are not attained. For service communications on the line network, a dedicated transmission band is therefore made available.